This invention relates to the provision of a coating on a metal substrate such as aluminum to enhance adhesive bonding to such substrate, and is particularly directed to the electrolytic deposition of a coating, particularly an organic coating, on a metal substrate such as aluminum, to form a chemical bond between the coating and the substrate, and providing a film for adhesive bonding of the substrate to a structural member e.g. of an aircraft, or to permit the metal substrate to be spot welded.
Primary adhesive bonding is being considered for an increasing number of aircraft structural joints. Thus, in view of the higher risks involved in the use of bonded primary joints, a need exists for optimum bond strength, long life, and economical and energy conservative processes. Generally, metal surfaces, such as aluminum, may be phosphoric acid oxidized, and this oxide layer becomes involved in adhesive bonding via van der Waals forces and/or hydrogen bonding. However, a stronger bond, such as a chemical bond, is desired to prevent a consequent possibility of reduced adhesive strength and failure due to environmental factors, such as high humidity and temperature.
The adhesion of polymers (as adhesives) to metal substrates is, generally, a complex process. Usually, the metal surface has to be pretreated, and the adhesive has to have sufficient activity for the mating of the two to result in an optimum bond. Substrates, e.g. metals, have to be prepared for this union of such two dissimilar materials as a polymer and a metal by first cleaning and then etching the metal, and, as in the case of aluminum, by anodizing (oxidizing) or other surface treatment such as the so-called FPL etch, employing a solution of sodium dichromate in sulfuric acid. These treatments have the effect of increasing surface area in order to enhance mechanical interaction between the polymer and the metal substrate. However, an important criterion for bonding of the coating to the metal is the wettability of the surface of the metal, and this is related to surface energy. Where mechanical interactions alone between the metal and polymer coating are involved, due primarily to surface roughening, the resulting bond strength is relatively weak.
It is accordingly highly desirable to provide a metal substrate or surface with a coating, and having a strong bond between the coating and the metal, so that upon subsequent application of an adhesive to the coating, the weak link is not the bond between the adhesive and the metal, but rather the weak link would be the adhesive per se. The result would be a higher bond strength between the metal substrate such as aluminum and the part which is adhesively secured to the metal, via the adhesive.
Further, if a metal surface such as aluminum is anodized, according to present practice, for adhesive bonding, the resulting oxide formed provides too high a surface resistance for spot weldability. If such surface is deoxidized to make it spot weldable, it cannot be adhesive bonded according to present practice. It accordingly would be beneficial if a metal surface could be prepared so that it could be either spot welded or adhesively bonded, or both.
Exemplary of the prior art are the following patents:
U.S. Pat. No. 3,026,255 discloses a method of protecting an oxidized metal surface by dipping the object into a solution of a polymer containing carboxyl, sulfonic or phosphonate groups.
U.S. Pat. No. 3,672,972 discloses a method for improving the adhesive bond between an aluminum surface and other materials, by anodizing the aluminum substrate in an acidic electrolyte.
U.S. Pat. No. 3,790,450 discloses the deposition of an organic coating on a metal substrate, by electrochemically coating a metal substrate as the positive electrode of an electrochemical cell, utilizing a non-aqueous electrolyte comprising a lower aliphatic acid, an aromatic compound and a Group III-A halide. It appears that an ionic polymerization occurs on a platinum anode, requiring a relatively long period of up to about 15 hours to form a polymerized carbonaceous residue.
U.S. Pat. No. 3,734,784 discloses anodic oxidation of an aluminum surface to form an active hydrated amorphous layer of aluminum oxide, followed by treatment of the activated aluminum surface by immersion in an alcohol-water solution of an organic compound containing a functional group such as a carboxyl group which is reactive with the hydrate on the aluminum surface, to form a water-insoluble product at the surface which provides a chemical bond between the overlying organic film and the aluminum substrate.
U.S. Pat. No. 3,990,953 discloses formation of elemental silicon as a corrosion resistant coating, employing a non-aqueous electroplating solution containing a non-aqueous solvent solution of a silicon halide or haloid silane, wherein the silicon compound is decomposed in an electric field to deposit silicon on a cathodic metal surface.
One object of the present invention is the provision of a coating on a metal surface, such as aluminum, having a strong chemical bond to the metal substrate. Another object is the provision of procedure for electrolytically depositing a coating on a metal substrate such as aluminum to provide a true chemical bond between the coating and the metal substrate, and permitting the metal substrate to be either adhesively bonded to another member or to be spot welded. A still further object is the elimination of the anodization process, ordinarily employed in present practice, for adhesively bonding aluminum. Yet another object is the provision of procedure to induce an organic compound or polymer to form a coating having a covalent or electrovalent bond with a metal substrate such as aluminum, the organic compound or polymer having functional groups to provide bondability, corrosion resistance and protection against bond degradation by humidity.